Garnarcho, who started the game but was substituted after 59 minutes with Leeds leading 2-0, faced criticism online after being partly at fault for the visitors’ second goal.
But Ten Hag said he is not worried about the 18-year-old’s form and insisted the critics have got it wrong.
“I think he is doing well, he has an impact on our game,” Ten Hag said. “I think he is a threat, [against Leeds] he had good actions, he was lively, created chances, but had to score.
“And finally, it’s about that, that you have an effect. As a striker, you have to at least be on the scoring list, the assist list, key action list to have the right impact.
“As a United player, we expect you to have an impact to influence the score, to influence the result, to positively influence the game by scoring goals and having an assist.”
Garnacho is enjoying an impressive breakthrough season at Old Trafford, making 22 appearances in all competitions and scoring two goals.
United are in talks about extending his contract, and Ten Hag said he is already shown that he has the confidence to make the grade.
“He’s quite emotional, that is his strength, he brings to his game, he wants to win, he wants to play football, he doesn’t want to miss any minute and I think that is a good thing,” Ten Hag added.
“He’s totally convinced about himself, that is a good thing, you need that in the biggest stress fractures that you have to perform and he is doing and he wants to contribute to the end.
“But he accepts decisions and the team is always above everything and I think he knows that. When he is coming off, that emotion, that frustration is not against that, that he’s not accepting it.”
Ten Hag is not expecting either player back for the trip to Leeds on Sunday and they remain doubts for the Europa League clash with Barcelona at Camp Nou on Thursday.
“I don’t expect [to have Martial or Antony available against Leeds] but we have some days,” Ten Hag said.
“I can’t say for 100% [they will miss Barcelona] but I don’t expect [they will be back].
“It’s a tough season, with the World Cup in between and a lot of load, overload, from the players, that’s quite clear. That process is already longer term coming on, that the load on the players is too much.”